Vacuum apparatuses are used in various industrial fields, such as semiconductor manufacturing and liquid crystal display manufacturing. Particularly in the semiconductor manufacturing and liquid crystal display manufacturing, processes such as film formation and etching are performed in a low-pressure atmosphere in a vacuum apparatus. The vacuum apparatus normally includes vacuum pumps so as to maintain a vacuum state or low-pressure state in vacuum containers for performing the processes and measurement.
The conventional vacuum pumps are roughly divided into a discharge type and a storage type. A pump of the discharge type draws a gas in through an inlet and discharges the gas through an exhaust outlet. The storage type draws a gas in through an inlet and stores the gas inside the pump. Generally, a storage-type pump can be evacuated to a point of high vacuum, but the quantity of gas that can be stored is naturally limited. Therefore, in a process that is performed at a reduced pressure with a gas always flowing, a storage-type pump is not suitable, but a discharge-type pump is actually employed.
Generally, a discharge-type pump having a higher ultimate vacuum has a higher exhaust rate and a lower allowable back pressure. Examples of vacuum pumps that operate in a molecular flow range with a high ultimate vacuum of 1.33×10−4 Pa (10−6 Torr) include turbo-molecular pumps, screw pumps, and oil-diffusion pumps. These pumps each have a high exhaust rate, regardless of the size, and a very low allowable back pressure of 133 Pa (1 Torr) or lower. Examples of pumps that have low ultimate vacuums and operate at a back pressure substantially equal to atmospheric pressure include Roots pumps, screw pumps, rotary pumps, and diaphragm pumps. Examples of pumps having medium ultimate vacuums include mechanical booster pumps and executor pumps.
In a vacuum apparatus, it is necessary to employ optimum vacuum pumps, depending on a required gas pressure, gas cleanliness, gas flow rate, gas type, vacuum container volume, or the like. Generally, if the gas pressure is as high as 40 Pa (300 mTorr), a single pump that operates with a back pressure substantially equal to atmospheric pressure can be employed. On the other hand, if the gas pressure is low, an exhaust system in which a pump that operates in a molecular flow range and a pump that operates with a back pressure equal to atmospheric pressure are connected in series is employed instead of the single pump. If the gas flow rate is high, a booster pump is interposed between the two pumps, so that the three pumps are connected in series and to exhaust gas.
In a mass-production factory of semiconductors or liquid-crystal displays, most of the processes required for production are performed at a reduced pressure. In such a case, a plurality of vacuum containers to be processed are integrally mounted on one device, so that a plurality of cluster tools that can transport substrates between the vacuum containers are aligned. This means that, generally, a plurality of vacuum containers are arranged together. In a conventional device, one independent exhaust system is provided for each one of the vacuum containers. The vacuum containers are in one-to-one correspondence with vacuum pumps, and each of the vacuum pumps evacuates only each corresponding one of the vacuum containers.
A vacuum pump that operates at a back pressure equal to atmospheric pressure requires a large power for rotating a rotor and consumes much more electric power, compared with a pump that operates at a low back pressure and has the same exhaust rate. Also, such a vacuum pump is large and heavy. In the conventional device, it is necessary to employ such large and power-consuming vacuum pumps in the same number as the number of vacuum containers. As a result, the total power consumption and the installation area of the device are large, and the production costs cannot readily be lowered.
Furthermore, since a vacuum pump that operates at a back pressure equal to atmospheric pressure has a lower ultimate vacuum on the suction side, there is a problem that, once an impurity gas adheres to the surfaces of wafers or the inner surfaces of the vacuum containers, the processing performance drastically deteriorates. Also, it is often difficult to place such pumps in the vicinity of the vacuum containers, because these pumps are too large in size. Therefore, the vacuum pumps need to be connected by long piping lines. This is a main reason for a decrease in processing rate or processing efficiency in a process that requires a large quantity of flow gas.
Also, the exhaust gas discharged from the vacuum containers used for semiconductor production might contain precipitant substances. As a result, solid substances adhere to the inner walls of the piping lines, and the exhaust conductance of the vacuum apparatus is greatly reduced.
In view of the above problems, the principal object of the present invention is to provide a vacuum apparatus that consumes less electric power and has a smaller installation area, and in which a large quantity of gas can flow without impurity gases entering vacuum containers from the exhaust system. Another object of the present invention is to provide a vacuum apparatus that has no impurity gases entering into vacuum containers, and can prevent a decrease in exhaust conductance due to a smaller cross-sectional area of a piping line even when the vacuum apparatus is used in a production process in which a precipitant exhaust gas is generated.